Drop Python 2.6 and 3.3

[nicoddemus]

Following pytest after its next release (3.3).

[hugovk]

Here's the pip installs for py from PyPI for the last month, showing low numbers for those two:

$ pypinfo --percent --pip py pyversion
python_version percent download_count
-------------- ------- --------------
2.7              58.7%      1,077,790
3.5              15.6%        287,337
3.6              14.6%        268,091
3.4              10.0%        183,970
2.6               0.9%         16,040
3.3               0.2%          4,083
3.7               0.0%            245
None              0.0%             49
3.2               0.0%             17

[nicoddemus]

Thanks for the numbers @hugovk